A Prediction about Time Travel ... and Other Speculations
... with an Update (2011/09/13).
Okay, we know that time windows are possible -- we've seen both Enigma and Agatha using them.
We have been told by Othar, a kinda-sorta-reliable narrator, that physical time travel is impossible because of the difference in energy levels between past and present. The energy requirement to transfer a physical object is just too great for any available power source.
According to Othar's Twitter journal, though, in the alternative future of a devastated Europa the (probably lethal) level of ambient radiation provides sufficient energy to make possible the transfer of a mind from future to past, but not a physical object.
A prediction: The Heterodyne Device/Lamp Thingie will turn out to be the key to providing the near-infinite amount of power required to make physical time travel possible. The bolt holes on its base will enable it to be attached to the rest of the Time Portal mechanism.
Time travel of physical objects opens up any number of opportunities for mischief ... and can solve some mysteries, too. Lucrezia may have used it to go back in time to the beginning of recorded history to establish the Cult of the Goddess among the Geisterdamen. And time travel could explain where Bill, Barry, and even Original Lucrezia are -- they're in transit from past to future, and will make their appearances at the end of the story.
And when the story is done, the Good Guys will prevent time travel from ever again being misused by Evil. The Lamp Thingie itself will be the very last thing transported back in time through the portal ... to the curio shop in Paris where Gil found it.
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► UPDATE
Good heavens. I just realized that time travel can clear up most of the mysteries about Higgs. When we hear the tale of how Airman Axel Higgs saved Klaus, he's a heroic but seemingly undistinguished fellow who suffers some severe wounds (broken arms and legs) during the crisis. (He looks pretty young here, doesn't he?)
Yet when we meet Higgs in Mamma Gkika's a short time later, he's completely healed. No explanation is provided for how he escaped from the hospital and wound up at Mamma Gkika's. After this point, Higgs seems to be well-nigh unkillable, shrugging off sword thrusts through his body. He also looks older, and when we see his bare chest, much later, it's covered with scars. What happened?
I've been assuming that Higgs was some kind of undercover Jäger. But what if Higgs really had been just an ordinary airman until he rescued Klaus? What if he was snatched out of the Great Hospital through a Time Portal by the Good Guys, was given some sort of medical treatment that granted him extreme hardiness, had various adventures with the Good Guys both in the future and in the past, and then was returned to Mamma Gkika's in Volume 8 to assist the Good Guys in the Castle?
This can explain why Higgs is aware of things that happened hundreds of years earlier, why Castle-in-Otilia recognized him, and why he knows his way around the interior of Castle Heterodyne. His extreme hardiness could be explained by his being a very "fresh" Jäger who still looks human. It would even explain why he's so imperturbable and why he doesn't seem to display much concern for Agatha's safety: Higgs already knows how the next dozen volumes of the story will turn out.